The present invention relates to a multi-layer gas sensor element and a gas sensor including the element. More particularly, the present invention relates to a multi-layer gas sensor element including a porous body, which element produces minimal warpage and does not suffer exfoliation, cracking, etc.; i.e., which element exhibits high durability; and a gas sensor including the gas sensor element, which sensor is highly resistant to poisoning by exhaust gas.
Conventional multi-layer gas sensor elements (hereinafter referred to simply as xe2x80x9celementsxe2x80x9d) have been employed in a gas sensor (such as an oxygen sensor, an HC sensor, or an NOx sensor) for detecting a specific component contained in exhaust gas of, for example, an internal combustion engine, and measuring the concentration of the component. In some cases, such a gas sensor element includes a porous body for covering a detection electrode which is brought into contact with a gas to be detected, in order to protect the detection electrode from poisoning. However, warpage of conventional multi-layer gas sensor elements including a porous body has not heretofore been satisfactorily prevented. Therefore, conventional gas sensor elements have tended to suffer cracking due to such warpage.
The present invention contemplates solving the above-mentioned problems, and an object of the present invention is to provide a multi-layer gas sensor element including a porous body, which element produces minimal warpage and does not suffer exfoliation, cracking, etc.; i.e., which element exhibits high durability; and a gas sensor including the gas sensor element, which sensor exhibits high durability. Al2O3 can be used as the ceramic component for realization of.
Accordingly, the present invention provides a multi-layer gas sensor element comprising a solid electrolytic member, a substrate, and a porous member, wherein each of the substrate and the porous member has a thickness greater than that of the solid electrolytic member with respect to a lamination direction; the substrate and the porous member face each other and sandwich the solid electrolytic member; a ceramic component constituting the substrate in the highest volume percent thereof is the same as the ceramic component constituting the porous member in the highest volume percent thereof; and the volume percent (R2) of the ceramic component contained in the porous member is 80% or more the volume percent (R1) of the ceramic component contained in the substrate. Characteristic features of the multi-layer gas sensor element reside in minimization of the differences in shrinkage percentage between the substrate and the porous member during firing and the difference in thermal expansion therebetween during use.
As another aspect of the invention, in the gas sensor element of the present invention, when the mean grain size of crystals constituting the substrate is referred to as xe2x80x9ca1xe2x80x9d and the mean grain size of crystals constituting the porous member is referred to as xe2x80x9ca2,xe2x80x9d the value A represented by the equation
A=a1/a2xe2x80x83xe2x80x83(1) 
preferably falls within a range between 0.9 and 5 inclusive, more preferably 1.0xe2x89xa6Axe2x89xa63.25, most preferably 1.0xe2x89xa6Axe2x89xa62.0. When the value A falls outside a range between 0.9 and 5 inclusive, warpage of the gas sensor element may fail to be satisfactorily prevented.